Chapter7:Rheological properties & sensory characteristic

Chapter7:Rheological properties & sensory characteristic of green tea yogurt during storage

184

1

2

3

4

5

6 CHAPTER VII

7 RHEOLOGICAL PROPERTIES AND SENSORY CHARACTERISTICS OF

GREEN TEA YOGURT DURING STORAGE

7.1 Introduction

Yogurt and related fermented dairy products have considerable economic

importance worldwide owing to their high nutritional values. For instance, most dairy

products usually contain substantial amount of highly bioactive compounds produced as a

result of enzymes breakdown of milk proteins (Allen, 1982). This strategic property can be

further diversified by adding nutraceutical (nutritional and pharmaceutical) ingredients such

as plant extracts rich in phytochemicals to the yogurt (Guggisberg, Eberhard, & Albrecht,

2007). In the present study green tea rich in polyphenolic compounds was used to fortify

yogurt, thus making herbal-yogurt a new fermented food category targeting consumers with

variety health issues associated with beneficial effects of green tea consumption (Narotzki,

Reznick, Aizenbud, & Levy, 2012). However, the presence of green tea affected the

fermentation of milk resulting in increased acidification and proteolysis (Section 5.31 and

5.3.7 respectively.). These are expected to alter the coagulation of milk protein and

subsequently the modification of textural and rheological properties of yogurt. Determining


185

the consistency of the yogurt gel network by rheological methods is a challenge because the

structure can be partly damaged in the act of sample preparation. In addition factors related

to yogurt formation i.e a) the heat treatment/homogenization of the milk base (casein/whey

protein ratio), b) the starter culture, and c) technological influences such as temperature,

pressure, valves and the shear history are also known to be an important determinant of

yogurt structure ( Sodini, Remeuf, & Haddad, 2004). The starter culture in turn may also be

influenced by the presence of green tea. The objectives of the present study were to

determine the influence of green tea supplemented yogurt on its sensory characteristics and

various physical and rheological properties, WHC, whey separation and total solid values

during storage.

7.2 Materials and Methods

Samples preparation and methods for experiments carried out in this section were as

described in Sections 3.12, 3.13, 3.14 and 3.15.

7.3 Results and Discussion

7.3.1 Effects of green tea on rheological characteristics of refrigerated yogurt

For rheological measurements yogurts were stirred, as consumers usually do before

consumption, prior to sampling. This is the most used method to perform viscometry or

oscillation tests on yogurt probably because it is difficult to find and standardize a

mechanical and reproducible method to stir yogurts without breaking substantial amount of

their structure (Vercet, Oria, Marquina, Crelier, & Lopez-Buesa, 2002). Manual slow

stirring is regarded as the most effective way to preserve yogurt structure and at the same

time allow preparation of yogurt sample that can be measured in a rheometer equipped with


186

parallel plate geometry (Vercet et al., 2002), suggested that when the yogurt network is

very weak, sedimentation of casein aggregates occur, and that the syneresis triggered by

this agglomeration leads to the formation of a depleted layer at the upper surface of the

sample. When oscillatory measures are to be taken in a horizontal geometry, this layer

causes slippage of the moving element, thus giving rise to an apparent reduction in

modulus.

7.3.1.a Apparent viscosity

The apparent viscosity of plain and green tea yogurts is shown in Figure.7.1. All

three yogurts showed shear thinning properties, demonstrated as decreased viscosity as the

rate of shearing increased (at higher shear rate the viscosity of the yogurt decreases making

it more runny). If time is considered as variable factor, the fluid may show rheopectic

behavior. This is generally due to a reversible change in the structure of the material with

time under shear, with a limiting viscosity ultimately being approached (Velez-Ruiz &

Barbosa Canovas, 1997).

Green tea yogurts showed a significant decrease in viscosity with time as compared

to plain yogurt at all storage periods studied. On the 1st day, the initial apparent viscosity of

plain yogurt (291.3 Pas) was higher than MGT and JGT-yogurts (86.41 and 76.98 Pas;

p<0.05; Figure 7.1). Although refrigerated storage had significant (p<0.05) effect on initial

apparent viscosity for green tea-yogurts (86.41-107.8, 76.98-115.7 Pas for MGT and JGT-

yogurts respectively; Figure 7.2, 7.3 and 7.4), but caused adverse effects on viscosity in

plain yogurt as demonstrated by the reduced viscosity on day 28 (230.8 ±1. 7 Pas)

compared to that in fresh yogurt (291.3 ±1. 64 Pas).Decreasing viscosity during storage is

not beneficial for industrial market and consumers acceptance.


187

Figure 7.1 Viscosities of fresh plain, Malaysia green tea and Japanese green tea-yogurt

0.1 1 10

1

10

100

Plain Yoghurt

Mal. Green T. Yoghurt

Jap. Green T. Yoghurt

Shear rate, (1/s)

Vis

cosi

ty,

(Pas

)

0.1 1 10 100

0.1

1

10

100

Plain-Day 1

Plain-Day 7

Plain-Day 14

Plain-Day 21

Plain-Day 28

Vis

cosi

ty,

(P

as)

Shear rate, (1/s)


188

Figure 7.2 Viscosity for plain yogurt during storage

Figure 7.3 Viscosity of Malaysian green tea yogurt during storage

0.1 1 10

1

10

100

Shear rate, (1/s)

Vis

cosi

ty,

(P

as)

Mal. Green Tea- Day1





0.1 1 10

1

10

100

Shear rate, (1/s)

Vis

cosi

ty,

(P

as)

Jap. Green Tea-Day1

Jap. Green Tea-Day7

Jap. Green Tea-Day14




189

Figure 7.4 Viscosity of Japanese green tea yogurt during storage

7.3.2 Dynamic rheological

7.3.2.a Frequency sweep

Yogurt is a viscoelastic material, so that its rheological behavior can be described by

two parameters; namely, the storage (G’; elasticity) and the loss (G”, viscosity) module

(Penna, Sivieri, & Oliveira, 2001; Ramirez-Santiago et al., 2010).

G’ is a measure of the energy stored and subsequently released per cycle of

deformation per unit volume. It is the property that relates to the molecular events of an

elastic nature. G” is a measure of the energy dissipated as heat per cycle of deformation per

unit volume. G” is the property that relates to the molecular events of viscous nature.

Another commonly used dynamic viscoelastic property, the loss tangent (which equals

G”/G’) denotes relative effects of viscous and elastic components in a viscoelastic

behavior.

Figure 7.5, 7.6, 7.7 and 7.8 shows the viscoelastic behavior of the fresh and

refrigerated plain- and green tea –yogurts during storage, which occurs in practice when

samples are taken out of the refrigerator for consumption and then stored again. Elastic

modulus dominating viscous modulus (G’>G”) showed that yogurt is having solid

behavior for all of the three yogurts type. As the frequency increases the G’ and G”

increases gradually. These results are in accordance to Sendra et al., (2010) which showed

yogurt’s predominantly elastic behavior (G′ > G″) over the whole range of frequencies

tested (0.01-10 rad/s) and which corresponds closely to that of a true gel. G’ higher than G”

can be due to decreased electrostatic repulsion and increased casein-casein interactions

(Rao, 2003). Fresh plain yogurt showed a higher range of G” and G’ than the green tea-

yogurts (31.34- 72.84, 12.73- 37.26 and 11.29 - 36.23 Pa, for PY , MGTY and JGTY


190

respectively ; p<0.05; Figure 7.5), which is showing the improvement of viscoelasticity of

the sample due to higher solid content.

There was no significant (p>0.05) difference in the Elastic modulus and viscous

modulus of plain and green tea yogurts during of storage. This indicates that the no

significant change in the bonding involved in the formation of gel structure by adding green

tea, which may be attributed to the function of phenolic compounds in green tea that no

interacted as a cross-link between protein molecules. Therefore, in green tea yogurt, the

nature of bonding did not change but the extent of cross-linking increased due to the

presence of dissolved phenolic compounds.

However, on 21th

day and 28 th

day of storage, plain yogurt showed statistically

different G” and G’ from that on 1th

day, 7th

and 14 th

day; however, the difference was

very small (p>0.05). This shows an increase in solid like characteristics of the samples on

storage for 2 weeks which may be due to increase in the acidity of the samples during

storage.


191

Figure 7.5 Frequency sweep of fresh plain yogurt, Malaysia green tea yogurt and Japanese green tea

yogurt

Figure 7.6 Frequency sweep for Malaysia’s green tea yogurt during storage.

0.1 1

10

100

–– G" Plain-Day1

– – G" Mal. Green Tea-Day1

–– G" Jap. Green Tea-Day1

Frequency (Hz)

Ela

stic M

od

ulu

s, G

' (P

a)

G' Plain-Day1

G' Mal. Green Tea-Day1

G' Jap. Green Tea-Day1

10

100

Vis

co

us M

od

ulu

s, G

" (Pa

)

0.1 1 10

1

10

100

–– G" Mal. Green Tea-Day1










Frequency (Hz)

Ela

stic M

od

ulu

s,

G' (

Pa

)

1

10

100

Vis

co

us M

od

ulu

s, G

" (Pa

)


192

Figure 7.7 Frequency sweep for Japanese green tea yogurt during storage

0.1 1

1

10

100






Frequency (Hz)

Ela

stic M

od

ulu

s, G

' (P

a)

Jap. Green Tea-Day1

Jap. Green Tea-Day7




1

10

100

Vis

co

us M

od

ulu

s, G

" (Pa

)


193

Figure 7.8 Frequency sweep for plain yoghurt during storage.

7.3.2.b Amplitude sweep

Amplitude oscillatory rheology has been used to characterize the rheological

properties of yogurt during the gel formation process (fermentation) without damaging the

weak gel network. Small deformation is defined as a small relative deformation (strain or

change in dimension) (e.g. ≤ 1%), which when applied does not disrupt the development of

the network structure, i.e., within the linear viscoelastic region. In this “linear” region, the

dynamic moduli are independent of the applied stress or strain (Lee & Lucey, 2010).

Figure 7.9 shows the values of amplitude sweep for fresh plain and green tea-

yogurts. The critical strain of fresh plain yogurt was the highest (44.07 - 16.58 Pa) followed

by JGTY and MGTY (27.9 - 6.95 and 24.24 - 7.69 Pa respectively) in range of 0.0004 -

0.10 strain. The elevated critical strains (yield point) suggest that the plain yogurt had a

higher resistance to deformation. The high elastic modulus (G’) for plain yogurt indicates it

0.01 0.1 1 10

10

100

– – G"Plain-Day1

– – G" Plain-Day7




G' Plain-Day1

G' Plain-Day7

G' Plain-Day14

G' Plain-Day21

G' Plain-Day28

Frequency, (Hz)

Ela

stic M

od

ulu

s, G

' (P

a)

10

100

Vis

co

us M

od

ulu

s, G

" (Pa

)


194

has more solid like and the binding particles (in this case milk protein) is packed closer than

those in green tea-yogurts.

While JGTY showed lower elastic modulus (G’) with initial strain (0.0004)

compared to plain yogurt suggested that it had lower stiffness and weaker than plain

yogurts. It had a slightly higher critical strain than Malaysia green tea yogurt hence it is

moderate in nature, followed by MGTY which had the lowest elastic modulus (G’) with

same strain compared with plain yogurt, suggested that it has less solid behavior (Figure

7.9). The differences in the storage modulus reflected the gelation characteristics within the

different yogurts.

Figure 7.10, 7.11 and 7.12 showed amplitude sweep of samples during storage. PY

showed the highest value of elastic modulus (G’) in 7th

day of storage (66.68- 28.73 Pa;

Figure 7.10) with the lowest value on the 28th day of storage (38.76- 15.59 Pa). Suggest

that in 7 day yogurt was more solid like and the particles (proteins) are closely packed.

Present of green tea to yogurt indicated no effect on yogurts during storage. The changes in

values of elastic modulus (G’) for both green tea yogurts were not significant .Except for

JGTY in 28th

day of storage which showed highest elastic modulus value (56.44- 23.56 Pa;

Figure 7.12). The lower value observed in yogurts fortified with the green tea can be

attributed to the production of exopolysaccharide by the green tea. The filaments of

exopolysaccharides interfere with the casein network. It can be assumed that protein strand

formation and protein–protein interaction is partly inhibited by the exopolysaccharides,

thus reducing the rigidity of the resulting yogurt gel.


195

Figure 7.9 Amplitude sweep of fresh plain, Malaysia green tea and Japanese green

tea- yogurts

Figure 7.10 Amplitude sweep for plain yogurt during storage

1E-3 0.01

10

20

30

40

50

60

70

80

90100

Yield Pt.= 0.0085

Yield Pt.= 0.0079

Yield Pt.= 0.0110

Strain

Ela

stic

Mo

du

lus,

G' (

Pa

)

Plain Yogurt

Mal. Green T. Yogurt

Jap. Green T. Yogurt

1E-3 0.01

20

40

60

80

100

Plain-Day 1

Plain-Day 7

Plain-Day 14

Plain-Day 21

Plain-Day 28

Strain

Ela

stic

Mod

ulus

, G' (

Pa)


196

Figure 7.11 Amplitude sweep for Malaysian green tea yogurt during storage

Figure 7.12 Amplitude sweep for Japanese green tea yogurt during storage.

1E-3 0.01

10

20

30

40

50

60

70

80

90100

Ela

stic

Mo

du

lus,

G' (

Pa

)

Strain

Mal. Green Tea-Day1

Mal. Green Tea-Day7

Mal. Green Tea-Day14



1E-3 0.01 0.1

10

100

Ela

stic M

od

ulu

s, G

' (P

a)

Strain

Jap. Green Tea-Day1

Jap. Green Tea-Day7





197

7.3.3 Effects of green tea on sensory evaluation of yogurt

The effects of adding green tea on sensory properties of yogurt samples are shown

in Figure7.13. Yogurt tasters play an important role in the quality assessment of samples.

Yogurt tasters’ parameters for evaluation included appearance, flavor, color, texture and

aroma.

The presence of green tea in yogurt increased yogurt appearance (7.1±1.5 and 6.

6±1.3 for MGT and JGT –yogurts respectively) compared to plain-yogurt (5.1±1.3),

indicating that both MGT and JGT yogurts were preferred by the panellists.

Texture score were lower for MGT and JGT –yogurts (6.4±1.7 and 6.1±1.9

respectively) compared to plain (7.4±1.7). Plain yogurt on the other hand showed low

flavor score (5.9±1.5) compared to MGT and JGT–yogurts (7.45±1.5 and 7.5±1.4

respectively). Colour has much impact on the acceptability of the green tea yogurt samples

(8.48±1.8, 8.0±1.7 and 5.9±2.2 for MGTY, JGTY and PY respectively). Green tea also

increased (p<0.05) the fresh yogurt score for aroma (8.9±1.3 and 8.3±1.5 for MGTY and

JGTY respectively) and the overall score (8.9±1.8 and 8.9±0.8 for MGTY and JGTY

respectively) compared to plain-yogurt (5.3±2.3 and 6.5±1.8 respectively).

In recent years, per capita consumption of yogurt has increased drastically because many

consumers associate yogurt with good health (Karaolis, Botsaris, Pantelides, & Tsaltas,

2013). Yogurt is characterized as a fermented milk product with a refreshing flavor, a

smooth viscous gel, and a slight sour taste (Bodyfelt, Tobias, & Trout, 1988).These sensory

properties in addition to appearance, flavour, texture, and overall quality offer quality

control criteria (Isleten & Karagul-Yuceer, 2006) and thus were used in the evaluation of

yogurt. The mean sensory characters scores for all samples were 5.0 or higher (Table 7.1).

The hedonic score of 5 corresponds to moderate liking of the samples, and the hedonic


198

score of 8 corresponds to very much liking of the samples. Both green tea yogurts were

liked moderately by the consumers for their texture, which were less than considered for

plain yogurt.

Since the flavour of yogurt is affected by the rate of acid production during the

fermentation process (Gallardo-Escamilla, Kelly, & Delahunty, 2005), the high score of

flavour in both green tea yogurts indicate that the addition of green tea contribute to more

acid production from conversion of lactose to lactic acid. This finding is important because

the addition of some lactic acid bacteria can affect flavour compounds.

Sensory evaluation showed that the plain (5.3±2.3) and the green tea yogurts

(8.9±1.3 and 8.3±1.5 for MGTY and JGTY respectively) scored significantly higher (P <

0.05) aroma measurement. Acetaldehyde was firstly reported by Pette & Lolkema (1950)

as the main aromatic compound in yogurt. During manufacture, production of this

compound is only highlighted when a certain level of acidification is reached (pH 5.0). The

maximum amount is obtained at pH 4.2 and stabilizes at pH 4.0. The production of

acetaldehyde and other flavour compounds by S. thermophilus and Lb.bulgaricus occurs

during yogurt fermentation and the final amount is dependent on specific enzymes which

are able to catalyse the formation of carbon compounds from the various milk constituents.

This result was consistent with changes in pH and titratable acidity values that is, yogurts

with high acidity contained the highest amount of acetaldehyde.Since the present of green

tea to milk increased acidity of yogurts (see Table 5.1 and 5.2) it was suggested that

decomposition of aroma compounds occurred more in green tea yogurts than plain yogurt

during fermentation.

Overall, significant difference was observed between green tea yogurts (8.9±1.8

and8.4±0.8 for MGTY and JGTY respectively) and plain yogurt (6.5±1.8) in the scores of


199

overall acceptability; even though no significant difference was observed for MGTY and

the JGTY samples.This indicated that addition of green tea affected the overall

acceptability of yogurt due to the higher score of colour, aroma as well as the acceptable

flavour and appearance.

Green tea yogurt has the potential to attract new yogurt consumers because the

incorporation of green tea into yogurt can enhance the therapeutic value of yogurt (see

Section 6.3.8). Green tea yogurt could provide novelty in the dairy foods market and help

consumers obtain nutritious food with added health benefits.

Figure 7.13 Sensory analysis results of plain and green tea yogurts.

7.3.4 Effects of green tea on water holding capacity (WHC) of yogurt during storage

Water holding capacity (WHC) is related to the ability of the proteins to retain water

Appearance

Flavor

colour

Texture

Aroma

Overall

Plain

MGT

JGT


200

within the yogurt structure (Wu, Hulbert, & Mount, 2001). The mobility of water

molecules in yogurt, as reflected in WHC values, can affect yield, sensory evaluation,

stability (in physical terms) and texture. In fact WHC is an essential quality parameter such

that the viscosity can be increased, gel-structures can be created or the physical stability can

be lengthened by changing the WHC (Mao, Tang, & Swanson, 2001). Thus changes in

WHC as a result of functional additive such as herb water extracts may modify the

properties of yogurt in a predetermined manner.

In this study, WHC measurements showed significant differences between green tea

and plain yogurt samples (Figure 7.14). The higher WHC was obtained for green tea yogurt

samples (30.2±1.2 and 31.25± 0.67 % for MGTY and JGTY respectively p<0.05) than

those obtained in plain yogurt (26.05± 1.5 %) in first day which was 2.92 and 3.02% higher

than that of plain yogurt. The WHC of plain and green tea yogurts increased during frist

week (34.4±1.6, 33.8±1.1 and27.71±0.46% for MGTY, JGTY and PY respectively on 7th

day of storage) and thereafter it decreased (27.48±1.75, 27.75±2.2 and 23.98±0.96 % for

MGTY, JGTY and PY respectively on 28th

day of storage).Decrease in WHC of yogurt

during storage was observed by Sahan, Yasar, & Hayaloglu (2008). Decrease WHC in

yogurts during storage is partly due to the unstable gel network of yogurts, in which the

weak colloidal linkage of protein micelles cannot entrap water within its three- dimensional

network (Donkor, Nilmini, Stolic, Vasiljevic, & Shah, 2007).However Parnell-Clunies,

Kakuda, Mullen, Arnott, & DeMan (1986) hypothesized that as the β-lactoglobulin

interacted with κ-casein, more covalent bonds were formed and larger micelle sizes might

cause steric hindrances; all resulting in lower WHC (covalent bonds decrease the number of

charged groups present in the gel network) during storage.


201

In yogurts, increased micelle size and increased whey-casein and casein-casein

interactions lead to a more porous gel, which could retain more water (Sodini, Montella, &

Tong, 2005; Lee & Lucey, 2004).

Figure 7.14 Water holding capacity of yogurt by adding green tea during storage

Values are mean±. SD. (n=3).

7.3.5 Effects of green tea on changes of syneresis on yogurt during storage

Syneresis is defined as gel contraction that occurs concomitantly with liquid/whey

expulsion and relates to the inability of the gel network to entrap all of the liquid phase.

Most consumers consider syneresis to be a defect (Lucey, Tamehana, Singh, & Munro,

%W

ate

r h

old

ing

cap

acit

y(W

HC

)

Day

MGT-yogurt

JGT-yogurt

Plain-yogurt


202

1998). When the casein particles rearrange in the gel network, whey expulsion is

spontaneous, as the gel shrinks without the application of some external force (Lucey,

2002).

The syneresis (%) of all 3 types of fresh and stored yogurt is given in Figure 7.15.

On the first day of storage the green tea yogurts showed the lower synersis (3.29±0.89 and

3.26± 0.97 %) than those in plain yogurt (3.56±1.1 %; p> 0.05). All yogurts showed an

increase in the amount of syneresis up to 28 days of storage (4.13±0.3, 4.18±1.0 and

4.91±1.3%; for MGTY, JGTY and PY respectively; p < 0.05) at 4 °C. These findings are in

agreement with those of Ramirez-Santiago et al. (2010); Zare, Boye, Orsat, Champagne, &

Simpson (2011) who showed an increase in the extent of syneresis in yogurts with

refrigerated storage. An increase in syneresis with storage time was observed in all yogurts.

Although the phenomena occurring during syneresis are not fully understood, it is agreed

that increased syneresis with storage time is usually associated with severe casein network

rearrangements (Van Vliet, Lucey, Grolle, & Walstra, 1997 ; Ramirez-Santiago et al.,

2010), that promote whey expulsion. Functionality of hydrocolloids in yogurt is

demonstrated by their ability to bind water, interact with the milk constituents (mainly

proteins), and stabilize the protein network, preventing free movement of water (Tamime &

Robinson, 1999).

Whey separation is known to be related to instability of gel network and thus the

loss in ability to entrap all the serum phase (Lucey, 2002). The rate of acidification is

instrumental in whey separation of yogurt in which the faster the rate the poorer the

network rearrangement during whey expulsion thereby resulting in lesser whey separation

(Castillo et al., 2006). The shorter time for green tea yogurts than plain yogurts to reach pH


203

4.5 (see section 5.3.1) may result in the reduction in whey separation in MGTY and JGTY

as compared to plain-yogurt.

Figure 7.15 Effects of Green tea extract on the syneresis of yogurt

7.3.6 Effect of green tea on total solid content in yogurt

The total solid (TS) content of yogurt samples showed in Figure 7.16.It was

observed from the results that the total solids content in the MGTY was in the range of 9-13

%, and JGTY was in the range of 8-13 % while PY was in the range of 7-13 %.

These TS values were much lower than those in commercial yogurts which have a

typical range of 14-15% (Tamime & Robinson, 1999). This is because yogurt prepared in

the present studies was without the addition of stabilizers or milk powder which commonly

practiced in the commercial preparation of set or stirred yogurt. The apparent decrease in

total solids with storage time (8.72, 8.44 and 6.97 % for MGTY, JGTY and PY

Spo

nta

ne

ou

s w

he

y se

pra

tio

n(%

)

Day

MGT-yogurt

JGT-yogurt

Plain-yogurt


204

respectively) in day 28 may be explained by moisture loss which had an advantage of

increased syneresis (Figure 7.14). The total solids content of the yogurt samples had a

significant effect on the degree of syneresis. Reduction of free water and increasing the

proportion of solids content, which occur during fermentation, are two main factors

decreased rates of wheying off in the samples with high total solids. This is agreeing with

the finding by Jaros, Partschefeld, Henle, & Rohm (2006); Amatayakul, Sherkat, & Shah (

2006); Mahdian & Tehrani (2007) who found that increase whey separation of yogurt

occurred only when the total solids were decreased. In this regard the addition of green tea

may be seen as advantageous in reducing the wheying off while yogurt is being kept

refrigerated.

Figure. 7.16 Changes in total solid content of yogurts during refrigerated storage

Tota

l So

lid (

%)

Day

MGT-yogurt

JGT-yogurt

Plain-yogurt


205

7.4 Conclusion

The addition of green tea (2 g), as a source of phytochemicals, to supplement

yogurts appear to decrease viscosity behavior, elastic modulus and amplitude sweep than

those found in plain yogurt but promoted avenue for increased sensory properties, with high

consumer acceptability. Increased WHC in green tea yogurts contributed to lower synersis

and higher total solid content. Both green teas itself are well known for their beneficial

health effects, and together with yogurt they may constitute a functional food with

commercial applications


206

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Chapter7:Rheological properties & sensory characteristic